Keystone is a type of limestone, or coral rag, quarried in the Florida Keys, in particular from Windley Key fossil quarry, which is now a State Park of Florida. The limestone is Pleistocene in age, and the rock primarily consists of scleractinian coral, such as Elkhorn coral and Brain coral.
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The Hurricane Monument, commemorating victims of the Labor Day Hurricane of 1935, and located at mile marker 82 on U.S. Route 1 near Islamorada, is constructed of keystone, as is the David W. Dyer Federal Building and United States Courthouse.